America’s two party system is highly stable, but its parties’ issue positions are not. Democrats and Republicans have changed sides on many subjects, including trade, civil rights, defense spending, and fiscal policy, and polarized on newer issues like abortion and gun control. Yet party position change remains poorly understood. In this book David Karol views parties as coalitions of groups with intense preferences on particular issues managed by politicians. He explains important variations in party position change: the speed of shifts, the stability of new positions, and the extent to which change occurs via adaptation by incumbents. Karol shows that the key question is whether parties are reacting to changed preferences of coalition components, incorporating new constituencies, or experimenting on “groupless” issues. He reveals that adaptation by incumbents is a far greater source of change than previously recognized. This study enhances our understanding of parties, interest groups, and representation.